In the practice of magnetic resonance phenomena, the application of rf radiation to a sample and the detection of resulting resonant signal is accomplished in a structure surrounding the sample which may be a helical coil, saddle coil, resonant cavity, or a bird cage resonator. The latter structure is the object of the present work, wherein it is desired to obtain resonant frequencies in a bird cage type structure to facilitate studies of diverse chemical constituents and/or spatial distributions of such constituents which may be appropriate to the sample.
A bird cage coil is frequently described as a ladder circuit, which closes on itself, wherein the current flow down the coil is distributed sinusoidally around it. It is often asserted that the bird cage coil is essentially a transmission line. As a tuned rf circuit, it is employed in nuclear magnetic resonance apparatus for either or both of the functions of rf excitation and signal detection.
The bird cage coil differs in essential manner from saddle coils, helices and like structures in that phase shifts between constituent current loops are employed to provide the proper current distribution. For the bird cage coil, there is a requirement that the phase shift be discretely distributed around the circumference of the coil from zero to 2.pi. (or 2.pi., where the mode number, k, is an integer). The phase shift of each element is quite frequency dependent and as a consequence, the bird cage coil is tuned at a discrete frequency to achieve the desired phase shift constraint. It is desirable to achieve a quadrature driven bird cage coil in order to maximize power efficiency upon transmission and signal-to-noise ratio during signal reception.
The bird cage coil is particularly well suited to large volume samples as are routinely encountered with apparatus for medical imaging and in vivo analytic spectroscopy. There has evolved a vast literature regarding birdcage coils. The seminal work is due to by Hales et al, J. Mag. Reds., vol. 63, pp. 622-628 (1985); and see, Troppo, J. Mag. Reds., vol. 82, pp.51-62 (1989); Pascone, et al, Mag. Reds. Imaging, vol. 9, pp.395-408 (1991); Joseph and Lug, IEEE Ter. Med. Imaging, vol. 8 pp. 286-294 (1989); Lifer, J. Mag. Reds., vol. 124, pp.51-60 (1997).
The bird cage structure may be regarded as a periodic structure which closes on itself Periodic elements of the structure produce phase shifts which must aggregate to some integer multiple of 2.pi. when summed over the closed loop. Geometrically, the resonator has cylindrical symmetry and it is desired that the rf current in the axial direction along the periphery of the structure be proportional to sin(k.theta.) and/or cos(k.theta.), where .theta. is the azimuthal angle about the cylindrical axis.
Birdcage coils are further delineated as balanced or unbalanced, quadrature phase sensitive/driven, high pass, low pass or band pass, which features may be served in the present invention by extension from prior art, although some of these features are better effectuated in the present invention.
In regard to balanced birdcage structures in the prior art, a balanced bird cage coil is driven (or signal derived) by coupling the signal ground to the midpoint of one (reactive) leg and the signal lead is suitably coupled to one end of the leg. This is described in U.S. Ser. No. 08/768,037.
The prior art includes a birdcage coil having N=4n legs (where n is an integer) operated in balanced quadrature mode by coupling the signal ground of the Q signal to the electrical midpoint of a first leg and the Q signal lead to the annulus proximate to the end of that first leg while the I signal ground is coupled to the electrical midpoint of a second leg and the I signal lead is coupled to the annulus proximate to the end of the second lead, where the first and second legs are relatively phase shifted by .pi.2. This prior art is described in U.S. Ser. No. 08/768,037, incorporated herein by reference. Although the number of legs is contemplated as an indefinite integer variable N, the special utility to be derived from a large number of legs is not further disclosed.
It is recognized that a large number of legs may contribute to increased RF magnetic homogeneity because at a field point, the larger number of contributing poles averages the direction and magnitude among the contributing sources to provide for diminished gradient within the space defined by the surrounding poles. See Hales, U.S. Pat. No. 4,694,255.
The utility of birdcage coils for small bore magnet systems has been examined by Cozier, et al, J. Mag. Reds., Series B 109, pp.1-1(1995), who found that the Q of the coil decreased with the increasing number of legs although homogeneity improved as expected. Eight and 12 legged coils were among the coil studied.
As magnet bore dimensions decrease, or as internal complexity of an NMR probe increases, the available space for discrete components, such as chip capacitors, becomes a constraint.
In the prior art, serial leg capacitances for a low pass birdcage coil were obtained in an integral leg construction by Liefer and Hartmann, U.S. Ser. No. 08/909,207.
Hales, U.S. Pat No. 4,694,255 has considered birdcage coils of as many as 32 legs, observing that the field uniformity is improved with the greater number of legs. However, Hales remarks that increasing the number of coil turns in an effort to increase homogeneity is not a viable solution since such increase would increase the inductance thereby limiting the resonant frequency and secondly, that reduction of open spaces between adjacent conductors would adversely effect the magnetic flux paths created by the current flow in the adjacent conductors.